This invention is directed to apparatus and a method for removing oil-coated mill scale from wastewaters, and more particularly, it is directed to magnetically removing oil-bearing suspended solids from recirculating cooling waters in a Hot Strip Mill (HSM).
Large quantities of water are used at Hot Strip Mill operations. For example water is used as a coolant to control the temperature of the work rolls. Controlling roll temperature improves both service life and product quality. If allowed to go uncontrolled, excessive roll temperatures tend to reduce surface hardness in the work rolls and produce increased roll surface wear. Increased bearing failures also occur because of excessive roll temperatures. As roll temperature increases, the work roll expands along its longitudinal axis and exerts lateral forces against the bearing mechanisms. The increased lateral forces cause the bearings to wear prematurely.
Overheated work rolls also reduce product quality. As pointed out above, excessive roll temperatures causes dimensional instability as the work rolls expand under the higher temperatures. In addition to expanding longitudinally, overheated rolls also expand diametrically and thereby reduce the gap between the opposed work rolls. The increased roll diameter, or reduced roll gap, produces a finished product that is thinner than the desired thickness or gauge.
Water is also used in high pressure jet sprays located within the HSM. The high pressure jet sprays blast scale from the surface of the raw, unfinished product prior to its rolling. The jet sprays clean the hot steel surface to remove any scale buildup to prevent scale from becoming imbedded within the product surface during the rolling operation. The jet blasted mill scale and water fall into a flume or sluice, located beneath the mill stands, where they are mixed with other mill wastewaters and sediments such as thick lubricating oils, chips and debris. The wastewaters carry the accumulated oily mixture of mill scale and solids to a collection point where the wastewater is filtered for recycling back into the HSM cooling water stream.
Hot Strip Mill sediments comprise a thick oily mixture of iron oxides, ferrous chips and lubricants. Such oil-bearing solids are very difficult to filter from the collected wastewaters because they clog all known filter matrices In an attempt to develop effective filtration of HSM wastewaters, pilot tests were conducted at a 68 inch wide Hot Strip Mill using an "ERIEZ"2500 gauss high intensity magnetic filter. It appeared reasonable that magnetic filtration would be suitable for removing ferrous mill scale from the wastewaters. However, the pilot scale tests proved disappointing. A portion of the wastewater short circuited the Eriez filter matrix and produced much lower removal efficiencies than expected. In addition, the thick oily mill scale mixture adhered to the filter matrix causing it to clog. This further reduced the removal rate by causing an even greater flow of wastewater to short circuit the matrix.
Various known backwashing procedures were tried in an attempt to purge the oily solids from the clogged filter matrix. The backwashing attempts proved to be ineffective because the thick oily solids clung fast to the matrix and could not be flushed out during backwashing. Eventually, the matrix became so clogged with oily solids that the filter had to be taken off line.
Such ineffective filtration practice can force oily mill scale and solids back into the HSM cooling water stream along with the recycled wastewater. As heretofore disclosed, these oily solids will cause premature wear and damage to the mill, and they will adversely affect product quality if they are rolled into the surface of the product.